Since February, Qihoo 360 has been rolling out security software to 200 million Windows XP users

Microsoft may have retired Windows XP, but one of China's leading security vendors is trying to keep the OS threat-free, and rolling out protection software to hundreds of millions of users in the nation.

On April 8, Microsoft officially ended support to the OS, meaning that Windows XP users will no longer receive security updates from the company. In preparation, Chinese security vendor Qihoo 360 has been developing its own free "XP Shield" software for Chinese PC users.

It has become one of the biggest projects for Qihoo 360, whose security products already have 470 million users. Since XP Shield was launched in late February, over 90 percent of the company's Windows XP users have downloaded the software, Sun Xiaojun, a security engineer with the company, said Wednesday. That puts its users at 200 million, he added.

Qihoo 360 has been among the security vendors in the country offering Windows XP protection software in the wake of Microsoft ending support to OS. In China, the operating system is still quite popular, and has a 70 percent share of the local market, according to Sun.

"We did a preliminary study and found that about two-thirds of Chinese users may not decide to upgrade their OS after Microsoft cuts support to Windows XP," he said in an interview.

Qihoo 360 and other rivals in China's security industry have tried to fill the void. Many Chinese are still accustomed to Windows XP and don't wish to upgrade because of the costs, Sun said. In the case of its XP Shield product, most of the users are regular PC consumers.

The company's software was put to the test earlier this month, when another Chinese security vendor held the "XP Challenge". Hackers were invited to find vulnerabilities in the XP security software from Qihoo 360, and two other Chinese companies, Tencent and Kingsoft. After a day of competition, only Qihoo 360's software came out unscathed.

Qihoo 360 plans to continue offering Windows XP security updates, even as Microsoft has been encouraging consumers buy Windows 7 and Windows 8. "We respect the user's choice, and will leave it up to users to decide on the OS," Sun said.

The company, however, faces a major challenge in keeping Windows XP secure over time. When Microsoft begins offering the next official security patches to Windows 7 and Windows 8, hackers will be able to analyze the updates to find weaknesses in the no-longer supported Windows XP OS, Sun said.

"If users don't have an effective security software, then they will face huge risks," he added.